Electronic information handling or computer systems, whether large machines, microcomputers or small and simple digital processing devices, require memory for storing data and program instructions. Various memory systems have been developed over the years to address the evolving needs of information handling systems. One such memory system includes semiconductor memory devices.
Semiconductor memory devices are rapidly-accessible memory devices. In a semiconductor memory device, the time required for storing and retrieving information generally is independent of the physical location of the information within the memory device. Semiconductor memory devices typically store information in a large array of cells. A group of cells are electrically connected together by a bitline, or data line. An electrical signal is used to program a cell or cells. The electrical signal on the data line is controlled by a driver circuit. Accordingly, a semiconductor memory device may include several groups of cells, each coupled together with a bitline operated by a driver circuit.
Computer, communication and industrial applications are driving the demand for memory devices in a variety of electronic systems. One important form of semiconductor memory device includes a non-volatile memory made up of floating-gate memory cells called flash memory. Computer applications use flash memory to store BIOS firmware. Peripheral devices such as printers store fonts and forms on flash memory. Digital cellular and wireless applications consume large quantities of flash memory and are continually pushing for lower voltages and higher densities. Portable applications such as digital cameras, audio recorders, personal digital assistants (PDAs) and test equipment use flash memory cards as the medium to store data, send and receive wireless faxes, and store digital audio clips and digital images. Each of these applications requires large amounts of highly reliable memory.
Prior to shipping, a manufacturer may test its semiconductor memory devices as part of a quality program to improve end-use reliability. One of the tests performed includes a bitline stress test. Generally, the bitline stress test is used to stress the materials of which the memory cells are made. In the bitline stress test, a higher voltage than usual is applied to all of the cells in a group. During such a test, a voltage is applied across the memory cell terminals while the memory cell is in a non-conductive mode. A leakage current may indicate a short within the memory device. Using a voltage for the bitline stress test that is higher than operating voltages may also simulate extended aging on the memory device. Thus, the higher-than-normal voltage may identify or even induce a short that might not otherwise affect device performance until after extended field use of the memory device. This short may be highly resistive and not affect the operation of the memory device. It may, however, degrade the operation over time. While the global stress test described above may identify a short within a memory device, it is generally incapable of identifying bitline-to-bitline defects with particularity.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate bitline stress test methods for semiconductor memory devices and circuitry to support such test methods.